The problem of passengers becoming trapped in an elevator in the event of a power failure has long been a concern. In the event of a power failure, unless the building is equipped with functional emergency generators, passengers will be trapped until power is restored, perhaps hours later. Being trapped in a crowded elevator can be uncomfortable, frightening, and potentially dangerous.
Buildings above 75 feet in height are required to have emergency generators with sufficient capacity to operate at least one elevator during a power failure. Elevator control systems typically have what is known as “Emergency Power Operation.” Even in buildings having functional emergency generators, the emergency power usually does not come on instantaneously. The power is typically interrupted for about 10 seconds. When the power is interrupted, the brakes are applied and the elevators abruptly stop, which can also be frightening and dangerous to riders. During a normal stop, the variable speed drive is used to ramp the speed of the elevator down until it is fully stopped, and then the brakes are applied as parking brakes. Emergency power does eventually allow the stopped elevators (one at a time) to evacuate their passengers down to the lobby before shutting down.
Power outages have two detrimental effects:    (1) When the power is lost, the elevators are subjected to voltage transients and mechanical operations that can cause the elevators to fault either electrically or mechanically. When emergency power is activated, those elevators that have faulted cannot be returned to service without intervention by trained elevator service personnel, leading to lengthy entrapment of passengers.    (2) The abrupt stoppage subjects passengers to negative accelerations that are not expected to exceed 1 g. However, a 1 g negative acceleration can cause people to fall and be injured. This is particularly true of elderly, handicapped, and infirm passengers.
It is desirable to eliminate or minimize the effects of power outages, or interruptions where emergency power is available, by allowing the elevator to continue running following a power outage until the next possible stop and stop normally rather than abruptly halting. This will minimize the chance of passenger injury or entrapment, reduce the possibility of a fault to the elevator electrical or mechanical systems, and leave the elevators in a condition that they can readily be placed back into service when the emergency generator comes on line or when power is restored.